Supreme Court strikes down Oklahoma grandfather clause in Guinn

The Supreme Court rules in Guinn v. United States on June 21, 1915, striking down Oklahoma's grandfather clause for voter registration. The rule exempted some white voters from literacy-test requirements if their ancestors had been eligible to vote before the Civil War. The Court says the law violates the Fifteenth Amendment because it uses ancestry to preserve racial exclusion from the ballot. Southern states continue using other devices, but Guinn becomes an early federal victory against racial voter suppression. States try to evade Black voting rights through formally clever rules after Reconstruction.